The present invention relates to screen printing apparatus comprising a circular screen with a squeegee within it, further to an impression cylinder for the circular screen, drive means and a system for detaching the squeegee during printing.
Screen printing apparatus of this kind are known and their circular screens are designed for discontinuous web printing or for sheet-fed printing.
In such procedures, when in a discontinuous web printing mode, the gap between printing end and printing beginning of the next print is minimal (because of the expensive printing materials). Commensurately, when in the sheet-fed mode, the distance between the front paper edge to the beginning of print also is minimal.
As regards optimal solutions of such problems with printing speeds becoming ever higher, increasing difficulties are encountered to move the squeegee in very short cycles (10 ms). Due to the interaction of different inks and patterns having different demands for inks, the apparatus requires extraordinary dynamics not easily controlled. Also, as regards circular screens, the ink runs through the screen in the stationary mode (open sites; the pattern is situated over the full periphery).
The present invention is directed toward a screen printing apparatus that allows very high operational speeds at short times of displacement, namely using separate cam disks. Such mechanical controls and appropriate designs markedly reduce the squeegee""s inertial forces.
Thanks to the cam disk, the squeegee pressure is generated mechanically. Mechanical generation of squeegee pressure substantially increases the speed, as compared to pneumatic controls.
The cam disk is preferably adjustable also in operation at its periphery with respect to the beginning of printing in order to attain the least spacing between printing end and beginning of printing. The effects of ink and of ink removal can be compensated by adjusting the cam disk. The adjustment or setting of the cam disk can be initiated, in the case of a central drive, by bevel gearing or by separate drives, for the circular screen and the squeegee (adjustment inside the synchronizing system).
The circular screen may comprise, in its null position, a closed zone to preclude ink leakage. Preferably, an ink-level regulator is used so that only minimal ink is needed in the screen.